The present invention relates generally to the field of liquid scintillation counting and in particular to improved methods for determining quench, volume and activity in a liquid scintillation flow system.
Liquid scintillation flow systems are well known in the art for providing an indication of the radioactivity present in a flowing sample. In such systems, the radioactive sample is mixed with an organic liquid scintillation medium and the resulting mixture is passed through a flow detector. The liquid scintillation medium emits light flashes or scintillations in response to nuclear disintegrations occurring within the sample. The intensity of a scintillation is proportional to the energy of the corresponding nuclear disintegration. A photodetector, such as a photomultiplier tube, within the flow detector detects the scintillations and provides output pulses having amplitudes proportional to the corresponding scintillations. In the liquid scintillation flow system, the pulses are counted in a plurality of pulse height channels or "windows" having upper and lower pulse height limits that together span a predetermined range of pulse heights. The counts for the windows may be plotted with respect to corresponding pulse heights to provide a pulse height spectrum representing the energy spectrum of the nuclear radiation emitted by the radioactive sample.
In order to determine the specific activity of the sample, it is necessary to determine values for three parameters, namely, the count rate of the sample, the volume of the sample producing the measured count rate, and the quench of the mixture flowing through the flow detector.
Most prior liquid scintillation flow systems have measured only count rate, uncorrected for sample quench or sample volume. Consequently, in such systems, it has been essentially impossible to accurately determine the specific activity of the sample.
With respect to mixture quench, it is well known in the art that "quench" refers to the decrease in the number of photons reaching the photodetector for a given nuclear disintegration in the liquid scintillation medium. For example, the production of photons in the scintillation medium may be decreased or emitted photons may be absorbed. In each case, the result is the reduction of the number of photons detected by the photodetector for a given nuclear disintegration. Because quenching decreases the number of photons applied to the photodetector, some scintillation events which would be detectable in an unquenched sample are below the photodetector detection threshold in a quenched sample. The result is that the number of counts per minute detected by the photodetector for a quenched sample is decreased as compared with an otherwise identical unquenched sample. The scintillation count rate detected in a quenched sample as compared to the disintegration rate of the sample is commonly referred to as "counting efficiency".
Quenching acts equally on all events produced by the same type of excitation particle, for example, electron (beta), alpha, proton, and so on. Thus, if quenching is sufficient to reduce the measured response for one disintegration by a given percentage, it will reduce all responses by the same percentage. In a liquid scintillation flow system, quenching results in a shift of the pulse height spectrum detected by the system to lower pulse height values, which is commonly referred to as "pulse height shift".
A few liquid scintillation flow systems have attempted to measure the quench of the mixture flowing through the flow detector. It is known, for example, to attempt to measure such quench by the use of the sample channels ratio method. However, such method is only accurate at high sample count rates, thus making the accurate quench determination of low activity samples essentially impossible. Moreover, prior art quench determination methods cannot be performed simultaneously with the measurement of sample count rate. Consequently, such methods require that sample count rate and quench be determined at different times. In a flow system, the count rate and quench characteristics of the sample flowing through the flow detector may change with time, rendering accurate quench determination difficult or impossible.
As noted above, a third parameter to be measured is the volume of sample which was measured by the liquid scintillation flow system. To determine such volume, it is known to use a cell within the flow detector having a predetermined volume. Using the volume per unit time flow rate of the system, the volume of the mixture and thus the sample may be determined. However, such a determination is affected by the accuracy to which the cell volume is measured and further by the accuracy to which such volume flow rate is determined. The accuracy of the volume flow rate may be affected, for example, by the accuracy of a pumping apparatus which delivers the mixture to the system. Consequently, inaccuracies in either the cell volume or the pumping system may affect the accuracy to which the specific activity of the sample may be determined. As used herein, specific activity means the disintegrations per unit time per unit volume of the sample.